1. Field of Invention
This invention relates to a system for the evaluation of oxygenator function, and in particular to a system for differentiating oxygenator malfunction from other causes of inadequate gas exchange by an oxygenator.
2. Discussion
A. A Description of Cardiopulmonary Bypass
Cardiopulmonary bypass (bypassing the heart and lungs) refers to the procedure of removing blood from the venous side of a patient's circulation and returning that blood to the arterial side of the circulation (usually to the aorta). During cardiopulmonary bypass, it is necessary to artificially perform the function of the patient's heart and lungs in order to maintain the health and life of the patient. It is the role of the oxygenator, or artificial lung, to perform the function of the lungs.
FIG. 1 is a schematic representation of the design of that part of a cardiopulmonary bypass circuit 10 performing the functions of blood removal, gas exchange and blood return. Venous blood 12A is continuously siphoned from the patient's body 14, usually from the right atrium of the heart, to a venous reservoir 16 via clear polyvinyl chloride tubing 18. This venous blood, which is low in oxygen content and high in carbon dioxide content, is subsequently pumped by a blood pump 20 through a venous blood inlet 22 of an oxygenator 24 and into a gas exchange compartment (not shown) of the oxygenator 24. After perfusing the gas exchange compartment, the blood 12B which is now oxygenated, exits the oxygenator 24 via an arterial blood outlet 26 and returns to the patient. Gas exchange occurs inside the gas exchange compartment of the oxygenator 24 such that the blood 12B returned to the patient has had oxygen added and carbon dioxide removed. The arrows in FIG. 1 represent the direction of blood flow.
Simultaneously with the delivery of blood to the oxygenator, a mixture of medical grade air and oxygen is delivered via an oxygenator ventilating circuit to a gas inlet 28 of the oxygenator 24. This gas ventilates the oxygenator (flows through the oxygenator) in a manner analogous to that which occurs in the human lung. That is, within the gas exchange compartment (not shown) of the oxygenator 24 oxygen from the ventilating gas diffuses into the blood perfusing the oxygenator 24, while carbon dioxide leaves the blood and enters the ventilating gas. The ventilating gas, now high in carbon dioxide content and lower in oxygen content, exits the oxygenator through a gas outlet 30. Generally, the concentration of oxygen in the patient's blood will be closely dependent upon the concentration of oxygen perfusing the oxygenator 24, and the concentration of carbon dioxide in the patient's blood will be closely dependent upon the total gas flow perfusing the oxygenator.
A representative design of a typical oxygenator ventilating circuit 32 (a circuit that delivers gas to an oxygenator) for cardiopulmonary bypass is shown in FIG. 2. Air 34 and oxygen 36 are delivered, under pressure, to the oxygenator ventilating circuit 32. The gases are usually piped in from the hospital's central gas supply, but can be delivered from gas tanks. The two gases then enter a blender 37 which reduces pressure, and proportions the gases to achieve the concentration designated by the perfusionist (the individual who operates the cardiopulmonary bypass machine). The blended gas then enters a flow controller 38 which allows the perfusionist to set the flow, in liters per minute, of gas to be delivered into the remainder of the ventilating circuit 32. From the flow controller 38, tubing 40 carries the gas to an anesthetic vaporizer 42 where a volatile anesthetic can be added to the gas flow. From the vaporizer 42, additional tubing 44 carries the gas mixture to a bacterial filter 46 and then on to the oxygenator 24. The gas mixture enters the oxygenator 24 through its gas inlet 28. The conduit for the gas, from flow controller 38 to oxygenator 24, is normally 0.25 inch clear, flexible polyvinyl chloride tubing 40, 44 and 48. Frequently, an oxygen analyzer 50 is placed in the ventilating circuit to assure that the gas exiting the blender 37 (and assumed to be flowing into the oxygenator 24) has the oxygen concentration designated by the perfusionist. A sensor portion 52 of the oxygen analyzer 50 is inserted into an adapter 54 which is usually positioned in the ventilating circuit prior to the bacterial filter 46.
Oxygenators in clinical use today are of two types. Bubble oxygenators are designed so that ventilating gas is bubbled directly through a reservoir of blood. Before the blood is returned to the patient, the bubbles are removed. Membrane oxygenators are designed to always keep the blood and ventilating gas separate. The membrane acts as a barrier which allows for exchange of gas molecules, but prevents the entry of gas bubbles into the blood.
B. The Causes of Hypoxemia
Various events occur during cardiopulmonary bypass which affect the concentration of oxygen and carbon dioxide present in the patient's blood. If the blood concentration of either of these gases varies from normal physiologic levels for even a short period of time, serious and permanent harm can result. The degree of harm will vary with the degree of insult, however, variations in the blood concentration of either gas for a short duration can result in permanent brain and neurologic damage, or even death. It is primarily the responsibility of the perfusionist, along with other members of the surgical team, to assure that the blood concentration of oxygen and carbon dioxide do not vary from normal physiologic levels during cardiopulmonary bypass.
Among the events that can alter the concentration of oxygen or carbon dioxide circulating in the patient's blood during cardiopulmonary bypass is a problem with the oxygenator. Such problems can be the result of an equipment malfunction or an operator error, both referred to henceforth as a malfunction. When a malfunction occurs, it is frequently not recognized until some resultant physiologic event, such as hypoxemia (an inadequate blood oxygen concentration), becomes evident. At that point in time, the cause of the problem still needs to be ascertained, and corrective measures taken quickly to insure the safety of the patient.
An oxygenator malfunction can involve, but is not necessarily limited to any of the following events, all of which compromise the oxygenator's ability to perform adequate gas exchange:
(1) a leak in the membrane (in a membrane oxygenator) allowing blood to enter the gas side of the gas exchange compartment;
(2) blood clot formation on the membrane;
(3) a crack in the oxygenator housing allowing gas to escape (to the atmosphere) before complete gas exchange occurs;
(4) obstruction of gas flow into or through the gas exchange compartment; and
(5) obstruction of gas flow out of the oxygenator gas exchange compartment, dangerously increasing the gas pressure within the gas exchange compartment. Hypoxemia can also occur despite a correctly functioning oxygenator due to other malfunctions which result in inadequate gas exchange by the oxygenator. These may involve unrecognized problems (unplanned alteration) with either the oxygen concentration or the flow of gas delivered to, and entering, the oxygenator. Gas supply malfunctions which can compromise attaining normal blood concentrations of oxygen and carbon dioxide include:
(1) oxygen concentration alteration secondary to a gas supply (34 and 36) malfunction 37, blender malfunction, or similar event;
(2) loss or absence of total gas flow secondary to an event such as disconnection of the tubing at any point of ventilating circuit 32;
(3) reduced gas flow secondary to a leak at any period in the ventilating circuit 32; and
(4) excessive gas flow from a blender 37 or flow controller 38 malfunction.
Additionally, the present inventor has discovered that a Venturi effect can result from a small break in the gas supply line 40, 44 and 48. This effect entrains room air into the oxygenator ventilating circuit 32, resulting in both excessive gas flow and a decrease in delivered oxygen concentration of said gas flow to the oxygenator secondary to room air dilution of the ventilating gas. Entrainment of room air into the ventilating gas is a previously unrecognized problem which can impair oxygenation of the patient. This problem of room air entrainment would be very difficult (and perhaps impossible) to detect by manual and/or visual inspection.
The frequency of oxygenator malfunctions is difficult to estimate for several reasons. First, because of the present inability to accurately monitor oxygenator function, events occur which are never recognized or accurately diagnosed. The present inventor has been informed by a representative of a major oxygenator manufacturer that of all oxygenators returned to the manufacturer for investigation of an intraoperative malfunction, only 30% are defective: The other 70% are free of defects. One can conclude therefore that those defect-free oxygenators were returned to the manufacturer because a problem other than oxygenator failure was misdiagnosed as a defective oxygenator.
Second, there is reticence on the part of practitioners to report untoward events and many experts in the field of cardiopulmonary bypass agree that numerous untoward events go unreported. (See Pierce, E., "Are Oxygenators (airplanes, oil spills, pesticides) Safe?", Ann. Thorac. Surg. 1989;48:467-468; Kurusz, M. et al., "Risk Containment During Cardiopulmonary Bypass", Sem. Thor. Cardiovasc Surg. 1990;2:400-409; Belcher, P., et al., "Hypoxemia During Cardiopulmonary Bypass", Ann. Thorac. Surg., 1990;50:336.) In spite of this, there are many published reports that demonstrate that the scenarios listed above are true problems, and not merely theoretical. (See Groom, R., "Do You Use A Gas Supply Oxygen Analyzer?", AACP Newsletter, 1991;7; Rubsamen, D., "Continuous Blood Gas Monitoring During Cardiopulmonary Bypass-How Soon Will It Be The Standard Of Care? ", J. Cardiothor. Anesth., 1990;4:1-4, Kurusz, M., et al., "Perfusion Accident Survey", Proceedings of Am. Ac. of CV Perf., 1986;7:57-65; Ditchik, J., et al., "Can we do without O.sub.2 analyzers?", Anesthesiology, 1984;61:629-30; McGarrigle, R., "General Anesthesia Without O.sub.2 Analyzer-A Substandard Practice.", Anesthesiology, 1985;63:116; Ghanooni, S., et al., "A Case Report Of An Unusual Disconnection.", Anesth. Analg., 1983;62:696-7; Dorsch, S. et al., "Use Of Oxygen Analyzers Should Be Mandatory.", Anesthesiology, 1983;59:161-2; Robblee, J., et al., "Hypoxemia After Intraluminal Oxygen Line Obstruction During Cardiopulmonary Bypass.", Ann. Thorac. Surg., 1989;48:575-6; Gravlee, G., et al., "Hypoxemia During Cardiopulmonary Bypass From Leaks In The Gas Supply System.", Anesth. Analg., 1985;64:649-50; Romanoff, M., et al., "Severe Hypocapnia During Cardiopulmonary Bypass.", Anesth. Analg., 1991; 72:410-11; Kubiak, D., et al., "Unusual Life-Threatening Hypercarbia During Cardiac Anesthesia And Cardiopulmonary Bypass.", J. Cardiotho. Anesth.., 1992;6:73-75; Maltry, D., et al., "Isofiurane-Induced Failure Of The Bentley-10 Oxygenator.", Anesthesiology, 1987;66:100-101; Cooper, S., et al., "Near Catastrophic Oxygenator Failure.", Anesthesiology, 1987;66:101-102; Dickinson, T., "Hypoxemia After Intraluminal Oxygen Line Obstruction During Cardiopulmonary Bypass.", Ann. Thorac. Surg., 1990:512-513; Kurusz, M., et al., "Oxygenator Failure", Ann. Thorac. Surg., 1990;49:511-513; Peirce II, E., "Are Oxygenators (airplanes, oil spills, pesticides) safe?", Ann. Thorac. Surg., 1989;48:467-468; Warren, S., et al., "Severe Hypoxemia During Cardiopulmonary Bypass.", Anaesthesia, 1986;41:1266-1267.)
The lack of ability to diagnose an oxygenator failure is disturbing to the clinician. Replacing an oxygenator during cardiopulmonary bypass is a difficult and dangerous maneuver in itself, and if the cause of poor oxygenation is due to an occult defect in the ventilating gas delivery circuit and not to a defective oxygenator, valuable time is wasted "changing out" the oxygenator unnecessarily. In such a situation, after the oxygenator exchange is completed, the patient remains hypoxemic and the true etiology of the problem is yet to be determined.
During cardiopulmonary bypass, the concentrations of both oxygen and carbon dioxide of blood entering 12A and leaving 12B the oxygenator (venous entering, arterial leaving) are routinely monitored. This monitoring can be performed either through intermittent sampling of arterial and venous blood samples with analysis of these samples performed on a standard blood gas machine, or with the use of an in-line continuous monitor such as CDI Extracorporeal Blood Gas Monitoring System manufactured by 3M Corporation. Therefore, the perfusionist knows how much oxygen is being taken up by blood (and how much carbon dioxide is being released by this blood) as the blood perfuses the oxygenator. Inadequate uptake of oxygen by the blood perfusing the oxygenator, or inadequate (or excessive) removal of carbon dioxide from this blood perfusing the oxygenator, indicates either 1) a malfunction of the oxygenator, or 2) a problem with gas delivery to the oxygenator. Unfortunately there is currently no device or technique that differentiates between these two problems. That is, there is no device presently available for differentiating oxygenator failure from a gas delivery problem.
The only monitoring device available to clinicians for monitoring gas delivery to the oxygenator is a standard oxygen analyzer which, as described previously, is usually positioned in the oxygenator ventilating circuit prior to the bacterial filter (See FIG. 2, 50). Importantly, an oxygen analyzer, regardless of where it is positioned in the gas delivery circuit, is incapable of detecting oxygenator malfunction, a total loss or absence of gas flow, reduced gas flow secondary to a leak in the circuit, or excessive gas flow. The oxygen analyzer is only capable of detecting an unplanned oxygen concentration change such as might be caused by a malfunction of the central gas supply 34 and 36 of the hospital, malfunctioning blender 37, or misadjusted blender 37, as long gas is flowing. If the malfunction involves cessation of gas flow into the oxygenator, as might occur from an accidental disconnection at any point in the ventilating circuit 40, 44 and 48, the oxygen analyzer is incapable of detecting this problem and, in fact, will indicate the oxygen concentration of the gas in that segment of the ventilating circuit where the oxygen sensor is positioned (although no gas is entering the oxygenator). Furthermore, if entrainment of room air (Venturi effect) occurs at a point in the circuit beyond the position of the oxygen analyzer sensor, the oxygen analyzer will not be able to detect the decrease in oxygen concentration of the gas (nor the associated excessive gas flow) being delivered to the oxygenator.
A flow controller (See FIG. 2, 38) positioned at any point in the gas delivery circuit merely establishes flow into the circuit immediately past the point of its position. It does not monitor flow beyond that position. Moreover, a flow controller does not monitor or guarantee appropriate gas delivery into the oxygenator and is incapable of detecting a disconnection of the ventilating circuit, a leak in the ventilating circuit, entrainment of room air through a defect in any component of the ventilating circuit, or oxygenator malfunction.
In summary, 1) oxygenator malfunctions and 2) malfunctions of the ventilating gas delivery circuit have been documented. Despite the published and unpublished knowledge of these events, and the long-appreciated need to assure appropriate oxygenator function, (See American Society of Extra-Corporeal Technology. "Suggested Pre-Bypass Perfusion Checklist," Perfusion Life, 7:17, 1990 and reprinted in Perfusion Life, 10:36, 1993) the present inventor is aware of no device, mechanism, technique, or system that has been presented or invented which can differentiate oxygenator malfunction from these other causes of hypoxemia. This lack of diagnostic capabilities for isolation of an oxygenator malfunction has resulted in extended and tragic delays in correction of that problem.
Thus it would be desirable to provide a system for isolating oxygenator malfunctions. In particular it would be desirable to provide a system which can differentiate oxygenator malfunction from other causes of hypoxemia thereby prevent the dangerous maneuver of unnecessary oxygenator exchange.
C. Oxygenator Pressure Abnormalities
FIG. 3A shows the gas outlet 56 of one type of membrane oxygenator 58 which provides the sole outlet for gas exiting the oxygenator membrane compartment 60. FIG. 3B shows another oxygenator 59 design having multiple concealed openings 62 which vent to the atmosphere (in addition to gas outlet 56). The oxygenator in FIG. 3A has certain advantages, but also some disadvantages.
In an oxygenator 58 designed with a single outlet for gas exiting the membrane compartment 60, positive pressure can develop on the gas side of the oxygenator membrane (not shown) if an occlusion of the gas outlet 56 occurs. If the pressure on the gas side of the membrane exceeds the pressure on the blood side of the membrane, gas is forced through the membrane and into the arterial blood where it may form bubbles. This condition can result in the patient receiving an air embolism, which can result in stroke, heart attack or other organ damage.
Conversely, if a negative pressure develops on the gas side of the membrane (which can develop with a malfunction of the exhaust gas evacuation system which is usually connected to gas outlet 56), gas would be sucked through the oxygenator and inadequate gas exchange would occur. (See Mushlin, P., et al., "Inadvertent Development Of Subatmospheric Airway Pressure During Cardiopulmonary Bypass", Anesthesiology, 1989;71:459-462.) This would result in hypoxemia.
Aware of the potential danger of a buildup in gas pressure, the American Society of Extra-Corporeal Technology recommends an inspection to assure that the gas exhaust pathway is unobstructed. (See American Society of Extra-Corporeal Technology, "Suggested Pre-Bypass Perfusion Checklist", Perfusion Life, 7:17, 1990 and reprinted in Perfusion Life., 10:36, 1993.) Thus it would be desirable to provide a system which can monitor the pressure in the gas compartment (the gas side of the membrane) of a membrane oxygenator.